Forced primary and forced secondary air biomass internal combustion top-lid updraft cook stove

ABSTRACT

The invention is a novel Top-Lit-Updraft cook stove with internal forced primary and secondary air, designed to burn biomass. The cook stove is specifically designed to cleanly burn biomass. The stoves design innovations include a cylinder-within-cylinder design to provide heated, forced secondary to the burner area right below the burner. The burner design addresses limitations in previous cookstove burner design by implementing a bluff body design to direct syngas to the secondary air jets, improving mixing and combustion of volatile gases.

BACKGROUND

The need for improved and clean cooking methods is still prevalent in many developing nations and areas of the world. Over 3 billion people still rely on a 3 stone fire or primitive cook stove for their daily cooking needs. This has resulted in significant health issues with users of these cooking methods. Approximately 2.6 million people die every year in developing countries from respiratory infections and other respiratory related medical conditions due to frequent exposure to unclean cooking emissions and particulate matter.

Many organizations and companies have been developing in this market for decades. However, the primary stoves developed have been designed to use wood, charcoal or coal as a fuel source. Reliance on these fuel sources have not relieved the issue of deforestation, mining and the initial cost of the fuel.

Two of the most widely disseminated stove types are the rocket stove and Top-Lit Updraft stove. The rocket stove is a simple stove which utilizes a natural draft induced by the heat to pull air through a bottom or side opening and into the combustion chamber. This type of stove also utilizes a side or bottom fuel loading opening, the air for combustion often enters through the fuel opening. These types of stoves can be very efficient and affordable, however to achieve efficiency the operator must be well trained and operate the stove carefully.

The Top-Lit Updraft type stove, called a TLUD, is named as such due to the nature of its operation. Fuel is loaded prior to operation and is not usually added after the combustion is initiated. The fuel is loaded into a combustion chamber and lit at the top. As the fuel burns the combustion zone moves vertically downward in the combustion chamber. Primary air for the combustion enters through openings in or near the bottom of the stove and provides oxygen for the initial combustion. This initial combustion is incomplete and produces syngas. A the top of TLUD stoves there are usually openings for secondary air or secondary air is assumed as the syngas mixes with environmental air. When the hot syngas mixes with secondary/environmental air complete combustion occurs. The stove design presented utilizes rice husk as its primary fuel source. Rice husk is a waste product of the rice milling industry and is separated from the rice grain at rice mills. There are very few marketable uses for rice husk, some include soil amendments, animal bedding and some construction related applications. There is over 100 million tons produced annually from the global rice industry and the few marketable uses leave a large excess which is usually dumped in fields or along roads. Rice husks have a very high silica content and much lower volatile matter than traditional fuels like wood or charcoal. To completely achieve complete combustion in a rice husk stove additional air is needed from a fan, blower or other forced air source. Many consumer rice husk stoves use a 12 volt computer fan or blower to provide this additional air. Consumer available rice husk stoves utilize the TLUD principles in a single cylinder stove design. The fan provides forced air through a grate on the bottom for only primary air. Rice husk TLUDs are capped at the top of the combustion chamber and usually have a ring of holes for syngas to move through, when the syngas moves through these hole and mixes with environmental air, the gas burns and secondary complete combustion is achieved. There are issues with the currently available technology that hinder the usability. The secondary combustion occurs outside the combustion chamber and is exposed to the environment. the flames are very susceptible to wind, rain and environmental temperatures. If primary combustion goes out then the stove top must be removed to reignite the fuel.

SUMMARY OF INVENTION

The subject invention comprises an improved Top-Lit Updraft cook stove to cleanly burn biomass fuel. The invention features internal combustion with forced primary air and force secondary air through a bottom plate and dual secondary air rings at the top of the combustion chamber. The invention employs a bluff body burner design to concentrate produced syngas and mix with secondary air. A physical air regulator valve controls air flow to primary and secondary air holes by increasing or decreasing pressure within the stove. This invention addresses many shortcomings of previous stoves by providing protection from environmental variables, usability and efficiency issues. This stove is specifically designed to burn rice husk and biomass, which requires very specific parameters for clean combustion.

DESCRIPTION OF FIGURES

FIG. 1 is an isometric view of the stove.

FIG. 2 is a cutaway view of the stove to show internal components.

FIG. 3 is an exploded view showing all parts.

FIG. 4 is an operations diagram.

DETAILED DESCRIPTION OF INVENTION

The invention described is a forced primary and secondary air biomass top-lit updraft cook stove. The stove is designed to cleanly burn rice husk as fuel, however the design allows for clean burning of other biomass fuels. The stove is a batch type stove, meaning fuel is loaded prior to use and cannot be added once combustion has been initiated. Once combustion is complete the stove must be emptied before reloading and the next use.

FIGS. 1-3 show all parts labeled as numbers, the same number will reference the same part on FIGS. 1-3, if that part is shown.

The burner assembly is comprised of part 7, the burner has integrated handles for removal from the stove. The top can be made of cast metal or sheet metal. Images 1-3 show a cast iron burner top. The burner has integrated pot supports to hold various sized pots. The pot supports hold the center bluff body to the burner frame. The bluff body directs hot pyrolysis gases toward the edges of the combustion chamber to facilitate better mixing with secondary air. A horizontally aligned handle and axial aligned handle are bolted to the burner top for ease of use.

The stove body comprised of parts 1, 8, 9, 12, 10, 7, 5, 13 and 11. The stove body is comprised of two cylinders, the outer cylinder, part 13, and the inner cylinder, part 8, which is also the combustion chamber. The bottom of the stove, part 10, the bottom grate of the inner cylinder, part 9 and the top ring that joins the top of the inner and outer cylinders, part 7, the parts are a tight fit that form an enclosed chamber for air to be pumped into. The parts are joined with spot welds. Primary air holes evenly distributed on the bottom grate and the 2 rows of secondary air holes are comprised of holes of the same diameter. The fan pumps air into the chamber between the inner and outer cylinder creating a pressurized volume. This pressurized air mass forces air evenly through each primary and secondary air hole, so that the total mass of air delivered through primary and secondary air holes is regulated by the number of holes in the bottom grate and the top of the inner cylinder, respectively. As the air regulator is opened allowing more air to enter the compressed volume an increased mass of air is pushed through the primary and secondary air holes, however retaining the same air ratio, so that complete combustion is achieved at low, medium or high power. This provides a solution to a problem of force primary, natural draft secondary air TLUD stoves. In the operation of these stoves the primary air is increased to increase power but the secondary air is not able to be adjusted to compensate for the increase in syngas produced, this shift allows the opportunity for unburned syngas to escape the stove and result in low efficiency and the release of unburned volatile gases. Parts 1 and 12 are high temperature insulation.

The air control valve comprised of parts 2, 3, and 4. The air control valve contains a 70 mm×70 mm 12 volt computer fan. The fan is run at a constant voltage and speed. 4 bolts hold parts 3 and 4 together with the fan in the middle. The power of the stove is controlled by the air control knob, part 2, as the knob is turned the air inlet is increased or reduced, increasing or decreasing the power of the stove, respectively. Part 4 is connected to the outer cylinder of the stove via a circular pipe connector and remains attached via a press-fit, however a screw hole is also available to insert a set screw if a more secure fit is desired.

The gasification process entails 2 stages, primary combustion and secondary combustion. Primary combustion takes place in the combustion chamber, part 8. The fuel is loaded into the chamber and lit at the top. Primary air flows through the bottom grate, part 9, and flows up through the fuel. As the fuel burns, the pyrolytic front moves down fuel column. As the biomass fuel pyrolyzes, the produced syngas flows upward in the combustion chamber. Secondary air flows up between the outer cylinder, part 13, and the combustion chamber, part 8, the secondary air then enters the combustion chamber through 2 rows of holes near the top. Directly below the burner, part 6, the syngas and secondary air mix and travel through the burner, achieving complete combustion. The burner, part 6, has a bluff body in the middle that concentrates syngas with the secondary air and facilitates fuel air mixing and allows for efficient operation.

Fuel is loaded into the inner cylinder which is the combustion chamber. A small pile of paper or tinder is lit at the top of the fuel. As soon as the fuel starts to burn, the pyrolysis process has begun, FIG. 4. The user then turns on the 12 volt computer fan and opens the air valve all the way open. The computer fan is located in the air control valve, part 3 houses the fan. After approximately 1 minute the user can put the burner top on and adjust the air valve to the desired cooking power and the stove is ready to be used. 

What is claimed:
 1. A forced primary and forced secondary air Top-Lit updraft internal combustion biomass cook stove.
 2. A cylinder-within-cylinder stove design to allow for one air source supplying both primary and secondary air by pressurizing a contained volume.
 3. A bluff body burner to facilitate mixing of syngas and secondary air.
 4. A design to facilitate internal secondary air and syngas mixing and combustion. 